Conventional techniques relating to wet process for producing copper powder by reducing copper ions in a liquid using a reducing agent include methods using hydrazine as a reducing agent (see Patent Literatures 1 to 3 below). Patent Literature 1 discloses a method including adding hydrazine or a hydrazine compound to a copper hydroxide slurry to form copper oxide and reducing the copper oxide to copper with hydrazine or a hydrazine compound. Patent Literature 2 proposes adjusting the pH of a copper salt aqueous solution to 12 or higher, followed by adding a reducing sugar, and followed by adding a hydrazine reducing agent. Patent Literature 3 describes a method including reducing a copper hydroxide slurry with a first reducing agent to obtain a copper suboxide slurry and reducing the copper suboxide slurry with a second reducing agent to obtain copper powder, in which hydrazine is used as a first reducing agent in combination with aqueous ammonia as a pH adjustor.
The assignee common to this patent application previously proposed another method including mixing an oily phase having a copper compound dissolved in an organic solvent and an aqueous phase having a reducing agent dissolved in water and reducing copper on the interface between the oily and the aqueous phase (see Patent Literature 4).
Patent Literature 5 describes still another method in which a reducing agent is added to a copper compound solution to produce fine copper particles by reductive precipitation, the method including (i) adding a reducing agent to a copper salt solution to produce independent monodisperse superfine copper nuclei and (ii) causing reductive precipitation of metallic copper from a copper salt solution in the presence of the superfine copper particles and a reducing agent.
Patent Literature 6 discloses a method including mixing copper (II) sulfate, ethylene glycol, and sodium hydroxide to form copper hydroxide, adding sucrose to the mixture, and heating the system to produce copper particles.